-thalassemia is a group of inherited blood disorders that result in defects in -globin chain production. Cooley's anemia (CA), or -thalassemia major, is the most severe form of the disease and occurs when an individual has mutations in both copies of the adult -globin gene. Patients with CA fail to make adult hemoglobin, have ineffective erythropoiesis, suffer from severe anemia, and are transfusion dependent for life. Currently, allogeneic bone marrow transplantation (BMT) is the only cure; however, few patients have suitable donors for this high morbidity/mortality procedure. This application outlines studies to establish a new BMT protocol that avoids the potentially lethal myeloablative conditioning of standard BMT. Preliminary studies have demonstrated the feasibility of using a non-cytoreductive conditioning regimen to rescue a preclinical humanized mouse model of CA from lethal anemia. These studies apply to several NHLBI Division of Blood Diseases and Resources stated goals: improving thalassemia treatment and management; development of hematopoietic transplantation; developing animal models for preclinical studies; and testing of novel cell-based therapies for the correction of thalassemia. The first main objective and specific aim of this project is to rescue humanized CA mice from lethal anemia by the use of non-cytoreductive bone marrow transplantation from a MHC-mismatched donor. The next main objective and specific aim is to determine the effect of adding an additional antibody treatment to the non- cytoreductive conditioning regimen. This study will employ a preclinical model of humanized CA by monitoring survival, HSC engraftment, disease and treatment-related pathology, and the donor marrow cell dose required after non-cytoreductive conditioned BMT. Together, these objectives would increase the clinical feasibility of utilizing bone marrow transplantation without cytoreductive conditioning to cure CA in human patients. Successful completion of the aims of this project will have a significant impact on the clinical management of CA patients. Furthermore, these ideas may be translatable to other hemoglobinopathies and blood disorders. The objectives in this proposal are the next step in developing a safe treatment for all patients with CA.